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Research linking vegetarianism to heart benefits doesn’t tell us what we really need to know

Posted Jan 31 2013 10:42am

Yesterday, someone sent me an email alerting me to this BBC news report of a study which found that vegetarians were found to be at lower risk of heart disease compared to non-vegetarians [1]. The study looked at about 44,500 adults living in England and Scotland over an average period of about 11½ years. Hospital admissions for heart disease (e.g. angina and heart attacks) were monitored, as were deaths due to heart disease. Vegetarians, it turned out, were 32 per cent less likely to suffer from heart disease-related issues than non-vegetarians. The authors put this apparent benefit, at least in part, down to factors such as lower blood pressure, cholesterol and body mass index in vegetarians compared to non-vegetarians.

If you’ve read this blog before, you’ll likely be familiar with the idea that studies of this nature (so-called ‘epidemiological’ studies) may tell us that two things are associated with each other, but not that one is causing the other. In this instance, we simply don’t know if the vegetarians had lower risk of heart disease because, say, they are generally more health-conscious than non-vegetarians. In this study, the researchers attempted to ‘control for’ (take into account) factors that might queer the pitch (known as ‘confounding factors’) such as smoking and physical activity.

However, the fact remains this can only ever be a quite imprecise science, and at the end of the day we’re still left with evidence which can only prove association (but not ‘causality’). The authors of this study do not mention this major limitation at all, and only give the briefest of mentions to the fact that their efforts to control for confounding factors might have come up short.

For me, there’s another glaring problem with this study: the fact that it focuses on heart disease only. The problem here is that heart disease only affects a minority of people, and it’s generally much more informative to take a wider view. If we really want to do this as objectively as possible then I suggest we should, wherever possible, assess the relationship of any lifestyle factor or intervention has with overall risk of death (sometimes referred to a ‘total mortality’ or ‘overall mortality’). This encompasses all illnesses and the diagnosis is never in doubt (unlike heart disease, where the diagnosis may not be clear cut at all).

We actually have evidence from the same group of researchers (based at Oxford University in the UK) that vegetarians do not have reduced mortality compared to non-vegetarians [2,3]. One of these studies [3] looked at death from heart disease specifically (as well as overall risk of death), and found it was not lower in vegetarians either.

But let’s not get too hung up on the Oxford researchers, and perhaps cast our net further afield. Last year a group of Chinese researchers performed a ‘meta-analysis’ [4] of 7 relevant studies [5-11] (including more than one from the Oxford researchers) which included data relating to diet and overall risk of death in almost 125,000 people. Remember, this is still epidemiological evidence and incapable of discerning cause and effect. This caveat noted, the meta-analysis did show a lower risk of death from heart disease and cancer incidence in vegetarians.

However, when it came to the all-important measure of overall risk of death there was no apparent protection afforded by a vegetarian diet.

It seems odd to me that the Oxford researchers would publish their latest study, when better evidence regarding the possible effects of a vegetarian diet is already available, including their very own evidence. Their focus has narrowed and, I think, risks people losing sight of the overall picture. I’d say the scope and relevance of the Oxford researchers’ work is on a distinctly downward slide.

References
1. Crowe FL, et al. Risk of hospitalization or death from ischemic heart disease among British vegetarians and nonvegetarians: results from the EPIC-Oxford cohort study. Am J Clin Nutr epub 30 January 2013

2. Key TJA, et al. Dietary habits and mortality in 11000 vegetarians and health conscious people: results of a 17 year follow up. Br Med J 1996;313:775-9

3. Key TJ, et al. Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). Am J Clin Nutr. 2009;89(5):1613S-1619S

4. Huang T, et al. Cardiovascular disease mortality and cancer incidence in vegetarians: a meta-analysis and systematic review. Ann Nutr 2012;60(4):233-40

5. Chang-Claude J, et al. Lifestyle determinants and mortality in German vegetarians and health-conscious persons: results of a 21-year follow-up. Cancer Epidemiol Biomarkers Prev 2005;14: 963–968

6. Key TJ, et al. Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). Am J Clin Nutr 2009;89:1613S–1619S.

7. Beeson WL, et al.Chronic disease among Seventh-day Adventists, a low-risk group – rationale, methodology, and description of the population. Cancer 1989;64:570–581

8. Thorogood M, et al.Risk of death from cancer and ischaemic heart disease in meat and non-meat eaters. BMJ 1994; 308: 1667–1670.

9. Berkel J, et al. Mortality pattern and life expectancy of Seventh-Day Adventists in the Netherlands. Int J Epidemiol 1983;12:455–459.

10. Ogata M, et al. Mortality among Japanese Zen priests. J Epidemiol Community Health 1984;38:161–166.

11. Key TJ, et al. Dietary habits and mortality in 11,000 vegetarians and health conscious people: results of a 17 year follow up. BMJ 1996;313:775–779.

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